The present invention relates to a picture coder, picture decoder, and picture transmission system that combine reliable inter-frame decoding with efficient use of bandwidth.
Inter-frame coding compresses data in, for example, systems that transmit digitized moving pictures through communication networks. Examples of such systems include videophone, videoconferencing, and video-on-demand (VOD) systems. Standards for inter-frame coding have been established by the Moving Picture Experts Group (MPEG) and the Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T), e.g. in ITU-T Recommendation H.261.
In these and similar standards, when a sequence of frames is transmitted, as in a moving picture, certain frames are coded as intra-frames, and the rest are coded as inter-frames. Intra-frames, also referred to as I-frames, are coded independently. Inter-frames, also referred to as predicted frames or P-frames, are coded in relation to a preceding reference frame, by coding only the differences between the inter-frame and the reference frame.
Inter-frame coding can greatly reduce the amount of coded data, but there is a problem. An inter-frame can be successfully decoded only if its reference frame has already been successfully decoded. If the reference frame is dropped (lost) or damaged (corrupted) in transmission, or if the vagaries of network communication cause the reference frame to arrive after the inter-frame, the inter-frame cannot be decoded. In the H.261 system, in which each inter-frame is coded with reference to the immediately preceding frame, the failure to decode even a single frame disables further decoding until the next intra-frame is received, and can lead to a lengthy episode of picture degradation.
As a solution to this problem, the present inventors have developed a picture transmission system in which the picture decoder notifies the picture coder of which frames were successfully decoded. The notification takes the form of a positive or negative acknowledgment signal transmitted from the picture decoder to the picture coder. Given such notification, the picture coder can quickly stop using reference frames that the picture decoder was unable to decode, thereby avoiding lengthy sequences of undecodable inter-frames. Relevant information can be found in U.S. patent application Ser. No. 08/710,405 and European Patent Application No. 96113836.9 (EP 0 763 944 A2).
This solution markedly improves the quality of moving pictures transmitted over unreliable communication networks and channels, but requires extra bandwidth for the transmission of the acknowledgment signals. In transmission systems with limited total bandwidth, one consequence is a diminished amount of bandwidth available for transmitting coded picture data from the picture coder to the picture decoder, forcing the picture coder to operate at a reduced frame rate. In other transmission systems, the bandwidth for transmitting the acknowledgment signals must be purchased at an extra cost.
In a variation of the above system, each frame is divided into two or more blocks, which are coded and decoded separately, and separate acknowledgment signals are transmitted for each block. This variation increases the probability of successful decoding, but also increases the amount of bandwidth required for transmitting acknowledgment signals. Moreover, as more acknowledgment signals are transmitted, the probability that an acknowledgment signal itself will be dropped or damaged in transmission increases.